A basic photosensor loop detection circuit for a phototransistor sensor device is shown and described in the 1990 Siemens Optoelectronics Data Book in General Photoelectric Application Circuits Application Note 36, page 11-104. The circuit disclosed therein has a low frequency signal filter for rejecting ambient light signals in the 120 H.sub.z range. High frequency light pulses emitted by, and used within the photosensor system are not filtered out. Certain unmodulated high frequency light signals detected by the sensor may also be passed into a feedback circuit and amplified by the high gain of a feedback transistor and a phototransistor. The amplified signal may cause oscillation in the photosensor loop circuit. Oscillation in the circuit reduces the sensitivity of the phototransistor thereby reducing the effectiveness of the photosensor. Oscillation may occur when the loop feedback becomes positive (less than -90.degree. or greater than -270.degree.). The Siemens ambient light filtering circuit induces a negative feedback into the phototransistor base circuit. This is accomplished by the feedback transistor which causes a 180.degree. negative phase shift and a feedback transistor base resistor and a feedback transistor bypass capacitor which combine to provide an additional 90.degree. of negative phase shift for a total of 270.degree. of negative feedback on the phototransistor base. However, because the phototransistor has parasitic base capacitance, certain detected intense light will cause an additional few degrees of negative phase shift to occur. This additional negative phase shift added to the -270.degree. already produced by the circuit results in some minimal positive feedback (greater than -270.degree.).